1. Field of the Invention
This invention relates to an image forming apparatus and more particularly to an image forming apparatus provided interchageably with a first unit for analogously forming an image by the electrophotographic copying process and a second unit for digitally forming an image by the thermal transfer printing process.
2. Description of the Related Arts
The concept of using a casing integrally incorporating therein a photosensitive member, a developing device, etc. as a unit and detachably fitting this unit inside a copying machine for the purpose of facilitating and conveniencing the maintenance, management, and repair of the copying machine has found recognition (for example, U.S. Pat. No. 4,627,701). Electrophotographic copying machines adopting this concept are now in popular use.
Today, in consequence of the growth of electronic devices, the practice of filing image data in the form of digital signals has been steadily disseminating and has come to urge the necessity for a printer which is capable of producing an image in response to a digital signal. When a printer capable of converting digital signals into images and an image reader capable of optically reading an original and generating a digital signal in response thereto are combined, the product of this combination offers the convenience that the printer can be additionally used as a copying machine. In cases where an image of a given document to be copied must be converted into digital signals in advance of the copying treatment, it is difficult to attain faithful reproduction of the image of the document. An attempt at attaining the faithful reproduction turns out to be a task in need of exceptionally high technique and immense cost.